Transparent, Thermally Stable Light-Emitting Component Having Organic Layers

ABSTRACT

The presently described subject matter relates to transparent and thermally stable light-emitting components having organic layers, and in particular to a transparent organic light-emitting diode having a charge carrier transport layer which is electrically doped with an organic dopant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/496,414, filed Sep. 19, 2005, which is a national phase ofand claims priority to International Application PCT/DE03/01021, filedMar. 27, 2003, which claims priority to German Patent Application DE 10215 210.1, filed Mar. 28, 2002, all of which are incorporated byreference in their entireties.

FIELD OF THE SUBJECT MATTER

The presently described subject matter relates to the organicsemiconductor technology concerning transparent organic light-emittingdiodes with doped charge carrier transport layers.

BACKGROUND

Ever since the demonstration, by Tang et al., 1987 [C. W. Tang et al.,Appl. Phys. Lett. 51 (12), 913 (1987)], of low operating voltages,organic light-emitting diodes (OLED) have been promising candidates forthe realization of large-area displays. They include a sequence of thin(typically 1 nm to 1 mu m) layers of organic materials, which can bevacuum-deposited or deposited from the solution, e.g., by a spin-onoperation. For this reason, these layers are often more than 80%transparent in the visible spectral region. Otherwise, the OLED wouldhave a low external light efficiency due to reabsorption. Contacting ofthe organic layers with an anode and a cathode is typically effected bymeans of at least one transparent electrode having, in many cases, atransparent oxide (e.g., indium tin oxide) and a metallic contact. Thistransparent contact (e.g., the ITO) can be located directly on thesubstrate. In the case of at least one metallic contact, the OLED as awhole is not transparent, but reflective or scattering (due toappropriate modifying layers, which do not belong to the actual OLEDstructure). In case of the typical structure with the transparentelectrode on the substrate, the OLED emits through the substratesituated on its lower side.

In the case of organic light-emitting diodes, light is produced andemitted by the light-emitting diode by the injection of charge carriers(electrons from one side, holes from the other side) from the contactsinto the organic layers situated there-between, as a result of anexternally applied voltage, the subsequent formation of excitons(electron-hole pairs) in an active zone, and the radiant recombinationof these excitons.

One feature of such organic components as compared with conventionalinorganic components (semiconductors such as silicon, gallium arsenide)is that it is possible to produce very large-area display elements(visual displays, screens). Compared with inorganic materials, organicstarting materials are relatively inexpensive (e.g., less expenditure ofmaterial and energy). Furthermore, these materials, because of their lowprocessing temperature as compared with inorganic materials, can bedeposited on flexible substrates, which opens up a wide variety of noveluses in display and illuminating technology.

The usual arrangement of such components having at least onenon-transparent electrode includes a sequence of one or more of thefollowing layers:

-   -   1. Carrier, substrate;    -   2. Base electrode, hole-injecting (positive pole), typically        transparent;    -   3. Hole-injecting layer;    -   4. Hole-transporting layer (HTL);    -   5. Light-emitting layer (EL);    -   6. Electron-transporting layer (ETL);    -   7. Electron-injecting layer;    -   8. Cover electrode, in most cases a metal having a low work        function, electron-injecting (negative pole);    -   9. Encapsulation, to shut out environmental influences.

The above structure represents one general case; in some cases somelayers are omitted (except 2, 5 and 8), or else one layer combinesseveral properties.

In the case of the above-described layer sequence, the light emissiontakes place through the transparent base electrode and the substrate,whereas the cover electrode includes non-transparent metal layers. Somematerials for the transparent base electrode include indium tin oxide(e.g., ITO) and related oxide semiconductors as injection contacts forholes (e.g., a transparent degenerate semiconductor). Used for electroninjection are base metals such as aluminum (Al), magnesium (Mg), calcium(Ca) or a mixed layer of Mg and silver (Ag), or such metals incombination with a thin layer of a salt such as lithium fluoride (LiF).

These OLEDs are usually non-transparent. However, there are applicationsfor which the transparency is of decisive importance. Thus, a displayelement may be produced which in the switched-off state appearstransparent, i.e., the surroundings behind it can be perceived, butwill, in the turned-on condition, provide the viewer with information.In this connection, one could think of car windshields or displays forpersons who must not be limited in their freedom of movement by thedisplay (e.g., head-on displays for surveillance personnel). Suchtransparent OLEDs, which represent the basis for transparent displays,are known, e.g., from

-   1. G. Gu, V. Bulovic, P. E. Burrows, S. R. Forrest, Appl. Phys.    Lett. 68, 2606 (1996);-   2. G. Gu, V. Khalfin, S. R. Forrest, Appl. Phys. Lett. 73, 2399    (1998);-   3. G. Parthasarathy et al., Appl. Phys. Lett. 72, 2138 (1997);-   4. G. Parthasarathy et al., Adv. Mater. 11, 907 (1997);-   5. G. Gu, G. Parthasarathy, S. R. Forrest, Appl. Phys. Lett. 74, 305    (1999).

In reference (1) above, the transparency is achieved by using thetraditional transparent ITO anode as a base electrode (that is, directlyon the substrate). Here, it should be mentioned that it is favorable forthe operating voltage of the OLED if the ITO anode is pretreated in aspecial way (e.g., ozone sputter, plasma incineration) in order toincrease the work function of the anode (e.g., C. C. Wu et al., Appl.Phys. Lett. 70, 1348 (1997); G. Gu et al., Appl. Phys. Lett. 73, 2399(1998)). The work function of ITO can be varied, e.g., by ozonization,ozone or oxygen plasma treatment, and/or oxygen-plasma incineration fromabout 4.2 eV to about 4.9 eV. In that case, it is possible to injectholes from the ITO anode into the hole transport layer in a moreefficient manner. However, this pretreatment of the ITO anode is mostlypossible if the anode is situated directly on the substrate. Thisstructure of the OLED is denoted as non-inverted, and the structure ofthe OLED with the cathode on the substrate as inverted. In (1), acombination of a thin, semitransparent layer, a base metal (magnesium,stabilized through the admixture of silver) and a conductive transparentlayer of the known ITO is used as a cover electrode. The reason why thiscombination is necessary is that the work function of the ITO is toohigh for electrons to be efficiently injected directly into the electrontransport layer and thereby make it possible to produce OLEDs having lowoperating voltages. This is avoided by means of the very thin magnesiumintermediate layer. Because of the thin metallic intermediate layer, theresulting component is semitransparent (transparency of the coverelectrode is about 50-80%), whereas the transparency of the ITO anodeconsidered as fully transparent is over 90%. In reference (1), anadditional ITO contact is deposited on the metallic intermediate layerby the sputter process, in order to ensure the lateral conductivity tothe connection contacts of the OLED surroundings. The consequence of theITO sputter process is that the metallic intermediate layer, in someembodiments, may not be thinner than 7.5 nm (1), as otherwise thesputter damage to the subjacent organic layers can be unacceptable.Structures of this type are also described in the following patents:U.S. Pat. No. 5,703,436 (S. R. Forrest et al.), applied for on Mar. 6,1996; U.S. Pat. No. 5,757,026 (S. R. Forrest et al.), applied for onApr. 15, 1996; U.S. Pat. No. 5,969,474 (M. Arai), applied for on Oct.24, 1997. Two OLEDs, one on top of the other, with the cathodesdescribed in reference (1), are described in reference (2). Here, agreen and a red OLED arranged one upon the other (“stacked OLED”) areprepared. Since both OLEDs are semitransparent, it is possible, throughsuitable voltages at the now 3 electrodes, to choose the emission colorin a targeted manner.

It is also known that an organic intermediate layer can be used toimprove the electron injection (references 3-5). In this case, anorganic intermediate layer is arranged between the light-emitting layer(e.g., aluminum tris-quinolate, Alq3) and the transparent electrode(e.g., ITO) used as a cathode. In some cases, this intermediate layer iscopper phthalocyanine (CuPc). This material is a hole-transport material(higher hole mobility than electron mobility). It exhibits high thermalstability. Thus, the sputtered-on cover electrode cannot do as muchdamage to the subjacent organic layers. An additional feature of thisCuPc intermediate layer is the small band gap (distance betweenHOMO—highest occupied molecular orbital—and LUMO—lowest unoccupiedmolecular orbital). Because of the low LUMO position, electrons can beinjected from ITO relatively easily. However, because of the small bandgap, the absorption in the visible region is high. For this reason, thethickness of the CuPc layer is limited to below 10 nm. Moreover, theinjection of electrons from CuPc into Alq3 or another emission materialis difficult, since their LUMOs lie generally higher. A furtherrealization of the transparent cathode at the top of the OLED wasproposed by Pioneer [U.S. Pat. No. 5,457,565 (T. Namiki), applied for onNov. 18, 1993]. In this case, a thin layer of an alkaline earth metaloxide (e.g., LiO2) is used instead of the CuPc layer. This improves theotherwise poor electron injection from the transparent cathode into thelight-emitting layer.

A further realization of the transparent OLED (G. Parthasarathy et al.,Appl. Phys. Lett. 76, 2128 (2000), WO Patent 01/67825 A1 (G.Parthasarathy), applied for on Mar. 7, 2001, provides for an additionalelectron transport layer (e.g., BCP=bathocuproine having a high electronmobility) in contact with the transparent cathode (e.g., ITO). There isan approximately 1 nm thick pure layer of the alkali metal lithium (Li)either between the light-emitting layer and the thin (e.g., 10 nm)electron transport layer or between the electron transport layer and theITO cathode. This Li intermediate layer drastically increases theelectron injection from the transparent electrode. This effect isexplained by a diffusion of the Li atoms into the organic layer andsubsequent “doping,” with the formation of a highly conductiveintermediate layer (e.g., degenerate semiconductor). Then, a transparentcontact layer (e.g., mostly ITO) is placed on the latter.

The above studies make the following points clear:

-   -   1. The choice of transparent electrodes includes ITO and similar        degenerate inorganic semiconductors.    -   2. The work functions of the transparent electrodes mainly favor        hole injection, but for this, too, a special treatment of the        anode is required, in order to further reduce its work function.    -   3. Previous worked was aimed at finding a suitable intermediate        layer which improved the injection of electrons into the organic        layers.

SUMMARY

The presently described subject matter relates to transparent andthermally stable light-emitting components having organic layers, and inparticular to a transparent organic light-emitting diode having a chargecarrier transport layer which is electrically doped with an organicdopant.

It was determined that employing dopants which can dope electrontransport materials (ETM) with a LUMO (of the ETM) less negative thanAlq3 are useful for a low voltage, high efficient and long lifetimetransparent OLED. Materials with a LUMO less negative than ETM-11 can beuseful.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-b show energy diagrams of the transparent OLED in one exampleembodiment.

FIG. 2 a shows OLED structures according to some embodiments of thedescribed subject matter.

FIG. 2 b is an energy diagram of a transparent OLED according to anotherexample embodiment.

FIG. 3 shows a luminance vs. voltage curve of Example 1.

FIG. 4 shows the normalized luminance over time in an accelerated agingtest of one device of the presently described subject matter. The figurecompares the bottom emitting device with the transparent device. Themeasured points almost completely overlap each other.

FIG. 5 shows the comparison of the optical transmittance of one deviceof the presently described subject matter (102) compared with otherdevices (103). The transmittance of the glass substrate with the ITOlayer is also shown for comparison purposes (101).

FIG. 6 shows the luminance vs. voltage curve of a transparent OLEDaccording to an embodiment with a non-inverted structure.

DETAILED DESCRIPTION

It is known, that for light-emitting diodes from inorganicsemiconductors, it is possible, through highly doped peripheral layers,to obtain thin space charge zones which, even in the presence of energybarriers, lead to efficient injection of charge carriers by tunneling.Here, the term “doping” includes the targeted influencing of theconductivity of the semiconductor layer through admixture of foreignatoms/molecules (as is possible for inorganic semiconductors). Fororganic semiconductors, the term “doping” includes the admixture, to theorganic layer, of specific emitter molecules; here, a distinction shouldbe made. The doping of organic materials was described in U.S. Pat. No.5,093,698, applied for on Feb. 12, 1991. However, in the case ofpractical applications of the described doping, this leads to problemswith the energy adaptation of the different layers and to reduction ofthe efficiency of the LEDs having doped layers.

In addition, electrical doping includes the phenomenon where a chargetransfer occurs from the HOMO (LUMO) of the n-dopant (p-dopant) to theLUMO (HOMO) of the n-type (p-type) semiconductor which transports thecharge carriers (also called matrix material). The charge density inequilibrium and the Fermi Level can be thus modified.

One object of the presently described subject matter is to provide afully transparent (e.g., 70% transmission) organic light-emitting diodethat can be operated at a low operating voltage, the organiclight-emitting diode having a high light-emission efficiency. At thesame time, the described subject matter includes the protection oforganic layers, in particular of the light-emitting layers, againstdamage during preparation of the transparent cover contact. Thedescribed subject matter includes stable components (e.g., operatingtemperature range up to 80 degrees C., long-term stability).

According to the presently described subject matter, some objects areachieved in combination with the following features: a transparent,thermally stable light-emitting component having the following sequenceof organic layers: a transparent substrate; a transparent anode; a holetransport layer adjacent to the anode; at least one light-emittinglayer; a charge-carrier transport layer for electrons; and a transparentcathode; in such a way that the hole transport layer is p-doped with anacceptor-type organic material and the electron transport layer isn-doped with a donor-type organic material, and the molecular masses ofthe dopants are greater than 200 g/mole.

The presently described subject matter further includes a transparent,thermally stable light-emitting component, having the following organiclayers: a transparent substrate; a transparent cathode; a chargetransport layer for electrons adjacent to the cathode; at least onelight-emitting layer; a charge-carrier transport layer for holes; and atransparent anode; in such a way that the electron transport layer isn-doped with a donor-type organic material and the hole transport layeris p-doped with an acceptor-type organic material, and the molecularmasses of the dopants are greater than 200 g/mole.

As described in Patent Application DE 101 35 513.0 (Leo et al.,submitted on Jul. 20, 2001), the layer sequence of the OLED can bereversed, thus the hole-injecting (transparent) contact (anode) can be acover electrode. As a result, in the case of inverted organiclight-emitting diodes the operating voltages can be considerably higherthan with comparable non-inverted structures. One reason for thisphenomenon is that the injection from the contacts into the organiclayers is less efficient, because optimization of the work function ofthe contacts in a targeted manner can be more difficult.

In the solution according to the described subject matter, the injectionof charge carriers from the electrodes into the organic layers (whetherhole- or electron-transporting layers) does not depend so strongly onthe work function of the electrodes itself. As a result it is alsopossible to use, on both sides of the OLED component, the same electrodetype, thus, e.g., two equal transparent electrodes, e.g., ITO.

The term side includes extending along a plane parallel to thesubstrate. The term bottom includes a position of a layer that is closerto the substrate than another layer. The bottom electrode includes anelectrode located somewhere between the substrate and at least oneorganic light-emitting layer. The term top includes a position of alayer that is further from the substrate than another layer. The topelectrode includes an electrode located somewhere not between thesubstrate and at least one organic light-emitting layer.

Some embodiments include a transparent, thermally stable light-emittingcomponent having organic layers, including a transparent substrate, atransparent anode, a hole transport layer adjacent to the anode, atleast one light-emitting layer, a charge-carrier transport layer forelectrons, and a transparent cathode, wherein the transparency in thevisible spectral region is at least 75%, wherein the hole transportlayer is p-doped with an acceptor organic material and the electrontransport layer is n-doped with a donor organic material, and themolecular masses of the dopants are each greater than 200 g/mole, andwherein the transparent, thermally stable light-emitting componenthaving organic layers is an organic light-emitting diode.

Some embodiments further include at least one of a hole-side blockinglayer located between the doped hole transport layer and thelight-emitting layer or an electron-side blocking layer located betweenthe doped electron transport layer and the light-emitting layer. Someembodiments further include a electrode layer located between the anodeand the hole transport layer and a electrode layer located between thecharge-carrier transport layer and the cathode.

In some embodiments, the doping concentration of the organic dopants issuch that an ohmic injection takes place from the anode into thecharge-carrier transport layer or from the cathode into the holetransport layer. In some embodiments, the electrode layers compriseindium tin oxide (ITO) or a degenerate oxide other than ITO. In someembodiments, the cathode includes a metallic intermediate layer adjacentto the subjacent doped, charge-carrier transport layer when the cathodeis located on top or the anode includes a metallic intermediate layeradjacent to the subjacent doped, hole transport layer when the anode islocated on top and wherein the metallic layer has a nominal thicknessbetween 0.1 nm and 3 nm.

In some embodiments, no metal layer is located between the doped holetransport layer and the anode when the anode is on top or between thedoped electron transport layer and the cathode when the cathode is ontop. The anode and cathode can be located between the substrate andencapsulation cover and the transparency can be at least 70% for eachwavelength between at least 400 nm and 800 nm. The molar concentrationof admixture in the hole transport layer or in the electron transportlayer or in both the hole transport layer and the electron transportlayer can be in the range of 1:100,000 to 1:10, calculated on the ratioof doping molecules to main-substance molecules. The molar concentrationof admixture in the hole transport layer or in the electron transportlayer, or in both the hole transport layer and the electron transportlayer, can be at least 1 wt %, calculated on the ratio of dopingmolecules to main-substance molecules.

In some embodiments, the thickness of each of the hole transport layeror the electron transport layer, of the light-emitting layer and of theat least one of a hole-side blocking layer or an electron-side blockinglayer lies in the range of 0.1 nm to 50 μm. In some embodiments, thecathode is in direct contact with a doped transport layer and is facingaway from the substrate when the cathode is on top or the anode is indirect contact with a doped transport layer and is facing away from thesubstrate when the anode is on top and wherein the doped transport layeris a hole transport layer or an electron transport layer. In someembodiments, the organic n-dopant material is selected from the groupconsisting of heterocyclic radicals, diradicals, dimers, an oligomer, apolymer, a dispiro compound, and a polycycle thereof, having thestructure according to one of the following formulae:

wherein structures 3 and 4 have one or more cyclic linkages A and/or A1and/or A2,

wherein A, A1 and A2 are selected from the group consisting ofcarbocyclic, heterocyclic, polycyclic ring systems, and any combinationthereof, which may be substituted or unsubstituted,

wherein A1 and A2 are present individually or together and A1 and A2 areselected as in structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S,and

wherein structure 7 has one or more bridge bonds Z and Z1, Z or Z1, Z1and Z2, or Z1 or Z2, and Z, Z1 and Z2 are independently selected fromthe group consisting of alkyl, alkenyl, alkynyl, cycloalkyl, sililyl,alkylsililyl, diazo, disulphide, heterocycloalkyl, heterocyclyl,piperazinyl, dialkyl ether, polyether, primary alkylamine, arylamine,polyamine, aryl, and heteroaryl.

The organic acceptor organic material can be a quiniode derivative or atriylidene derivative, with a reduction potential in the range of 0V vs.Fc/Fc+ to 0.4V vs. Fc/Fc+. In some embodiments, the n-doped, donororganic material is an asymmetrically substituted phenanthroline withthe following structure

wherein:R1 and R2 are selected from the group consisting of substituted orunsubstituted Aryl, Heteroaryl, and Alkyl; andR3 is selected from the group consisting of H, CN, substituted orunsubstituted Aryl, Heteroaryl, and Alkyl;R4 is selected from the group consisting of H, CN, COOR with R=Alkyl,Heteroalkyl, Aryl or Heteroaryl, substituted or unsubstituted Aryl,Heteroaryl, Alkyl mit C1-C20, and Cycloalkyl mit C3-C20.

In some embodiments, the n-doped, donor organic material has thestructure:

wherein M is selected from the group consisting of Ti, Zr, Hf, Nb, Re,Sn and Ge,each R is independently selected from the group consisting of hydrogen,C₁-C₂₀-Alkyl, C₁-C₂₀-Alkenyl, C₁-C₂₀-Alkinyl, Aryl, Heteroaryl,Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl, —OR_(x),—NR_(x)R_(y), —SR_(x), —NO₂, —CHO, —COOR_(x), —F, —Cl, —Br, —I, —CN,—NC, —SCN, —OCN, —SOR_(x), SO₂R_(x), and where R_(x) and R_(y) areselected from the group consisting of C₁-C₂₀-Alkyl, C₁-C₂₀-Alkenyl, andC₁-C₂₀-Alkinyl.

The n-doped, donor organic material can have the structure:

wherein R₁, R₂, R₃, and R₄ are independently selected from the groupconsisting of H, halogen, CN, substituted or unsubstituted aryl,heteroaryl, alkyl, heteroalkyl, alkoxy, and aryloxy.

In some embodiments, the anode is between the substrate and the at leastone light-emitting layer. In some embodiments, the cathode is betweenthe substrate and the at least one light-emitting layer. The electrodelayers can include different transparent contact materials.

Some embodiments further include a contact-improving layer locatedbetween the electron transport layer and cathode and a contact-improvinglayer located between the anode and the hole transport layer, whereinthe contact-improving layers are configured not to prevent charge frompassing through. Some embodiments further include a contact-improvinglayer located between the electron transport layer and cathode or acontact-improving layer located between the anode and the hole transportlayer, wherein the contact-improving layers are configured not toprevent charge from passing through. The light-emitting layer caninclude a mixed layer of several materials. The p-doped hole transportlayer can include a mixture of an organic main substance and an acceptordoping substance and an acceptor doping substance and the molecular massof the dopants can be greater than 200 g/mole. The electron transportlayer can include a mixture of an organic main substance and a donordoping substance and an acceptor doping substance and the molecular massof the dopants can be greater than 200 g/mole. In some embodiments, whenthe transparent cathode is on top, the transparent cathode includes atransparent protective layer or when the transparent anode is on top,the transparent anode includes a transparent protective layer. In someembodiments, when the transparent cathode is on top, the transparentcathode includes a metallic intermediate layer adjacent to the subjacentdoped charge-carrier transport layer or when the transparent anode is ontop, the transparent anode includes a metallic intermediate layeradjacent to the subjacent doped hole transport layer,

-   -   wherein the transparency of the metal intermediate layer in the        visible spectral region is at least 75% and the thickness of the        metal intermediate layer is between 0.3 nm and 3 nm. The        sequence of p-doped hole transport layer and transparent anode        can be repeated. The sequence of n-doped electron transport        layer and transparent cathode can be repeated. Some embodiments        further include a metallic electron-injection-promoting layer        located between the doped electron transport layer and either        the electron-side blocking layer or the light-emitting layer,        wherein the transparency of the metallic        electron-injection-promoting layer in the visible spectral        region is at least 75%.

In some embodiments, the top transparent contact layer (which is facingaway from the substrate) is in direct contact with the doped transportlayer, which doped transport layer is a hole transport layer or anelectron transport layer.

In some embodiments, the transparent organic light-emitting diodeincludes a thin (e.g., 1 to 10 nm thick) doped charge transport layer atthe interface with the top electrode (this layer being localized betweenthe light-emitting region and the electrode); the dopant concentrationbeing greater than 40 wt %, in some embodiments greater or at least 50wt %. In some embodiments, the transparent organic light-emitting diodeincludes a thin (e.g., 0.5 nm to 3 nm) pure dopant layer as a bufferlayer at the interface with the top electrode (between the chargetransport layer and the top electrode).

In some embodiments, ohmic injection occurs when the dependence of thecurrent with the applied voltage is linear (e.g., can be measured insingle carrier type devices (e.g., hole only devices)). In someembodiments, if a line fit (I=F(V)) to the I-V curve fits to at least95% in a range of at least 1 V (layer thickness of at least 50 nm) thenthe injection is ohmic. For a layer thinner than 10 nm, a dopantconcentration greater than or equal to 5%, perhaps greater than or equalto 10% may be required. For layers thicker than 10 nm, the concentrationmay be higher than 0.2%, perhaps higher than 1%, and if the layer isunder the top electrode, then the doping concentration may be higherthan 5%.

The cause of the increase of conductivity can be an increased density ofequilibrium charge carriers in a layer. Here, the transport layer canhave higher layer thicknesses than is possible with undoped layers(e.g., 20-40 nm), without drastically increasing the operating voltage.Similarly, the electron-injecting layer adjacent to the cathode can ben-doped with a donor-type molecule (e.g., an organic molecule orfragments thereof, see Patent Application DE 102 07 859.9). Thisn-doping leads to an increase in the electron conductivity due to higherintrinsic charge-carrier density. The transport layer can also be madethicker in the component than would be possible with undoped layers,since that would lead to an increase in the operating voltage. Thus,both layers are thick enough to protect the subjacent layers againstdamage during the production process (e.g., sputter process) of thetransparent electrode (e.g., formed from ITO).

In the doped charge-carrier transport layers (holes or electrons) on theelectrodes (anode or cathode), a thin space charge zone may be createdthrough which the charge carriers can be injected in an efficientmanner. Because of the tunnel injection, the injection is not hinderedby the very thin space charge zone, even in case of an energeticallyhigh barrier. The charge-carrier transport layer can be doped by anadmixture of an organic or inorganic substance (e.g., dopant). Theselarge molecules are incorporated in a stable manner into the matrixmolecule skeleton of the charge-carrier transport layers. As a result, ahigh degree of stability is obtained during operation of the OLED (e.g.,no diffusion) as well as under thermal load.

In Patent Application DE 100 58 578.7, filed on Nov. 25, 2000 (see alsoX. Zhou et al., Appl. Phys. Lett. 78, 410 (2001)), it is described thatorganic light-emitting diodes having doped transport layers show anefficient light emission when the doped transport layers are combinedwith blocking layers in an appropriate manner. Hence, in an embodiment,the transparent light-emitting diodes are also provided with blockinglayers. The blocking layer can be located between the charge-carriertransport layer and a light-emitting layer of the component, in whichthe conversion of the electric energy into light takes place. Theelectric energy of the charge carriers can be injected by current flowthrough the component. According to the described subject matter, thesubstances of the blocking layers can be selected so that when voltageis applied in the direction of the operating voltage, because of theirenergy levels, the majority charge carriers (HTL side: holes, ETL side:electrons) are not too strongly hindered at the doped charge-carriertransport layer/blocking layer interface (e.g., low barrier), but theminority charge carriers are efficiently arrested at the light-emittinglayer/blocking layer interface (e.g., high barrier). Moreover, thebarrier height for the injection of charge carriers from the blockinglayer into the emitting layer can be small enough that the conversion ofa charge-carrier pair at the interface into an exciton in the emittinglayer is energetically advantageous. This prevents exciplex formation atthe interfaces of the light-emitting layer, which reduces the efficiencyof the light emission. Since the charge-carrier transport layers canhave a high band gap, the blocking layers can be chosen to be very thin.In spite of this, no tunneling of charge carriers from thelight-emitting layer in energy conditions of the charge-carriertransport layers is possible. This permits obtaining a low operatingvoltage despite blocking layers.

One embodiment of a transparent OLED according to the described subjectmatter includes the following layers (non-inverted structure) (FIG. 2a):

-   -   1 Carrier, substrate;    -   2 Transparent electrode, e.g., ITO, hole-injecting        (anode=positive pole);    -   3 p-Doped, hole-injecting and transporting layer;    -   4 Thin hole-side blocking layer made of a material whose band        positions match the band positions of the layers enclosing it;    -   5 Light-emitting layer (possibly doped with emitter dye);    -   6 Thin electron-side blocking layer of a material whose band        positions match the band positions of the layers enclosing it;    -   7 n-Doped electron-injecting and transporting layer;    -   8 Transparent electrode, electron-injecting (cathode=negative        pole);    -   9 Encapsulation, to shut out environmental influences.

Another embodiment of a transparent OLED according to the describedsubject matter includes the following layers (inverted structure) (FIG.2 a):

-   -   1 Carrier, substrate;    -   2 a Transparent electrode, e.g., ITO, electron-injecting        (cathode=negative pole);    -   3 n-Doped, electron-injecting and transporting layer;    -   4 a Thin electron-side blocking layer of a material whose band        positions match the band positions of the layers surrounding it;    -   5 a Light-emitting layer (possibly doped with emitter dye);    -   6 a Thin hole-side blocking layer of a material whose band        positions match the band positions of the layers surrounding it;    -   7 a p-Doped hole-injecting and transporting layer;    -   8 a Transparent electrode, hole-injecting (anode=positive pole),        e.g., ITO;    -   9 Encapsulation, to keep out environmental influences.

The described subject matter includes structures with one blockinglayer, because the band positions of the injecting and transportinglayer and of the light-emitting layer can match one another on one side.Furthermore, the functions of charge-carrier injection and ofcharge-carrier transport into layers 3 and 7 may be divided amongseveral layers, of which at least one (namely that adjacent to theelectrodes) is doped. When the doped layer is not directly located onthe respective electrode, then layers between the doped layer and therespective electrode may be thin enough that they can efficiently betunneled through by charge carriers (e.g., 10 nm). These layers can bethicker when they have a higher conductivity (the bulk resistance ofthese layers may be smaller than that of the neighboring doped layer).The intermediate layers can then be considered to be a part of theelectrode. The molar doping concentrations can lie in the range of 1:10to 1:10000. The dopants can include organic molecules having molecularmasses above 200 g/mole.

The n-dopant, or dopant donor, can include a molecule or a neutralradical or combination thereof with a HOMO energy level (e.g.,ionization potential in solid state) more positive than −3.3 eV, or morepositive than −2.8 eV, or more positive than −2.6 eV and its respectivegas phase ionization potential is more positive than −4.3 eV, or morepositive than −3.8 eV, or more positive than −3.6 eV. The HOMO of thedonor can be estimated by cyclo-voltammetric measurements. Analternative way to measure the reduction potential is to measure thecation of the donor salt. The donor can exhibit an oxidation potentialthat is smaller than or equal to −1.5 V vs Fc/Fc+ (Ferrum/Ferroceniumredox-pair), or smaller than −1.5 V, or smaller than or equal toapproximately −2.0 V, or smaller than or equal to −2.2 V. The molar massof the donor can be in a range between 200 and 2000 g/mole, or in arange from 300 and 1000 g/mole. The molar doping concentration is in therange of 1:10000 (dopant molecule:matrix molecule) and 1:2, or between1:100 and 1:5, or between 1:100 and 1:10. Sometimes dopingconcentrations larger than 1:2 can be applied, e.g., if largeconductivities are required. The donor can be created by a precursorduring the layer forming (e.g., deposition) process or during asubsequent process of layer formation. The above given value of the HOMOlevel of the donor refers to the resulting molecule or molecule radical.

A p-dopant, or dopant acceptor, can include a molecule or a neutralradical or combination thereof with a LUMO level more negative than −4.5eV, or more negative than −4.8 eV, or more negative than −5.04 eV. TheLUMO of the acceptor can be estimated by cyclo-voltammetricmeasurements. The acceptor can exhibit a reduction potential that islarger than or equal to approximately −0.3 V vs Fc/Fc+(Ferrum/Ferrocenium redox-pair), or larger than or equal to 0.0 V, orlarger than or equal to 0.24 V. The molar mass of the acceptor can be inthe range of 200 to 2000 g/mole, or between 250 and 1000 g/mole, orbetween 300 g/mole and 1000 g/mole. The molar doping concentration canbe in the range of 1:10000 (dopant molecule:matrix molecule) and 1:2, orbetween 1:100 and 1:5, or between 1:100 and 1:10. Sometimes, dopingconcentrations larger than 1:2 can be applied, e.g., if largeconductivities are required. The acceptor can be created by a precursorduring the layer forming (e.g., deposition) process or during asubsequent process of layer formation. The above given value of the LUMOlevel of the acceptor refers to the resulting molecule or moleculeradical.

An n-dopant of the following structure can be employed in thetransparent p-i-n OLED:

where M is a transition metal, e.g., Mo or W; and where

-   -   the structural elements a-f can include: a=CR₉R₁₀, b=CR₁₁R₁₂,        c=CR₁₃R₁₄, d=R₁₅R₁₆, e=CR₁₇R₁₈ and f=CR₁₉R₂₀, where R₉R₂₀        independently of one another are hydrogen, C₁C₂₀ alkyl, C₁C₂₀        cycloalkyl, C₁C₂₀ alkenyl, C₁C₂₀ alkynyl, aryl, heteroaryl, NRR        or OR, where R=C₁C₂₀ alkyl, C₁C₂₀ cycloalkyl, C₁C₂₀ alkenyl,        C₁C₂₀ alkynyl, aryl or heteroaryl, where R₉, R₁₁, R₁₃, R₁₅, R₁₇,        R₁₉=H and R₁₀, R₁₂, R₁₄, R₁₆, R₁₈, R₂₀=C₁C₂₀ alkyl, C₁C₂₀        cycloalkyl, C₁C₂₀ alkenyl, C₁C₂₀ alkynyl, aryl, heteroaryl, NRR        or OR, or    -   in the case of structural elements c and/or d, C can be replaced        by Si, or    -   optionally a or b or e or f is NR, with R=C₁C₂₀ alkyl, C₁C₂₀        cycloalkyl, C₁C₂₀ alkenyl, C₁C₂₀ alkynyl, aryl, heteroaryl, or    -   optionally a and f or b and e are NR, with R=C₁C₂₀ alkyl, C₁C₂₀        cycloalkyl, C₁C₂₀ alkenyl, C₁C₂₀ alkynyl, aryl, heteroaryl,    -   where the bonds a c, b d, c e and d f, but not simultaneously        a-c and c-e and not simultaneously b-d and d-f, may be        unsaturated,    -   where the bonds a-c, b-d, c-e and d-f may be part of a saturated        or unsaturated ring system, which may also contain the        heteroelements O, S, Se, N, P, Se, Ge, Sn, or    -   the bonds a-c, b-d, c-e and d-f are part of an aromatic or        condensed aromatic ring system, which may also contain the        heteroelements O, S, Si, N,    -   where the atom E is a main group element, selected from the        group C, N, P, As, Sb,    -   where the structural element a E-b is optionally part of a        saturated or unsaturated ring system, which may also contain the        heteroelements O, S, Se, N, P, Si, Ge, Sn, or    -   the structural element a E-b is optionally part of an aromatic        ring system, which may also contain the heteroelements O, S, Se,        N.

The dopant can have the following structure II:

Suitable n-dopant precursors include the heterocyclic radicals,diradical, a dimers, an oligomer, a polymer, a dispiro compound or apolycycle thereof, having the structure according to the followingformulae:

where structures 3 and 4 have one or more cyclic linkages A and/or A1and/or A2, where A, A1 and A2 may be carbocyclic, heterocyclic and/orpolycyclic ring systems, which may be substituted or unsubstituted;

where A1 and A2 may be present individually or together and A1 and A2are as defined for structures 3 and 4 and T=CR22, CR22R23, N, NR21, O orS;

where structure 7 has one or more bridge bonds Z and/or Z1 and/or Z2,and Z, Z1 and Z2 may independently be selected from alkyl, alkenyl,alkynyl, cycloalkyl, sililyl, alkylsililyl, diazo, disulphide,heterocycloalkyl, heterocyclyl, piperazinyl, dialkyl ether, polyether,primary alkylamine, arylamine and polyamine, aryl and heteroaryl;Organic n-dopant compounds include the heterocyclic radicals ordiradicals, the dimers, oligomers, polymers, dispiro compounds andpolycycles of:

where the bridges Z, Z1 and Z2 can be independently selected from alkyl,alkenyl, alkinyl, cycloalkyl, silyl; alkylsilyl, diazo, disulfide,heterocycloalkyl, heterocyclyl, piperazinyl, dialkylether, polyether,alkylamine, arylamine, polyamine, Aryl and heteroaryl; X and Y can be O,S, N, NR₂₁, P, or PR₂₁; R₀₋₁₉, R₂₁, R₂₂ and R₂₃ are independently chosenfrom substituted or unsubstituted: aryl, heteroaryl, heterocyclyl,diarylamine, diheteroarylamine, dialkylamine, heteroarylalkylamine,arylalkylamine, H, F, cycloalkyl, halocycloalkyl, heterocycloalkyl,alkyl, alkenyl, alkinyl, trialkylsilyl, triarylsilyl, halogen, styryl,alkoxy, aryloxy, thioalkyl, thioaryl, silyl and trialkylsilylalkanyl, orR₀₋₁₉, R₂₁, R₂₂ and R₂₃, are part of a (hetero)aliphatic or(hetero)aromatic ring system alone or in combination.Preferred n-dopants are those with the structure:

where R1 is methyl or isopropyl and R2 is phenyl or cyclohexyl.

Illustrative examples of suitable organic n-dopants include thefollowing dimer structures, their diradical state and their monomer:

Other examples include (ED-9)2,2′-diisopropyl-4,5-bis(2-methoxyphenyl)-4′,5′-bis(3-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole;(ED-10)2,2′-Diisopropyl-4,5-bis(2-methoxyphenyl)-4′,5′-bis(4-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole;(ED-11)2,2′-Diisopropyl-1,1′,3,3′-tetramethyl-2,2′,3,3′,4,4′,5,5′,6,6′,7,7′-dodecahydro-2,2′-bibenzo[d]imidazole;(ED-8)2,2′-Diisopropyl-4,4′,5,5′-tetrakis(4-methoxyphenyl)-1,1′,3,3′-tetramethyl-2,2′,3,3′-tetrahydro-2,2′-biimidazole;(ED-12)2-Isopropyl-1,3-dimethyl-2,3,6,7-tetrahydro-5,8-dioxa-1,3-diaza-cyclopenta[b]naphthene;(ED-13) Bis-[1,3-dimethyl-2-isopropyl-1,2-dihydro-benzimidazolyl-(2)];(ED-14)1,1′,2,2′,3,3′-hexamethyl-4,4′,5,5′-tetraphenyl-2,2′,3,3′-tetrahydro-1H,1′H-2,2′-biimidazole;

Electron transport materials (ETM) which can be used as host for then-dopants include phenanthrolines, metal quinolinates, metalquinoxalinates, diazapyrenes and others.

Asymmetrically substituted phenanthrolines are described in the Europeanpatent application EP07400033.2. Asymmetrically substitutedphenanthrolines which can be used as ETM can have the followingstructure

where:R1 and R2 are chosen from substituted or unsubstituted Aryl, Heteroaryl,Alkyl;R3 is chosen from H, CN, substituted or unsubstituted Aryl, Heteroarylor Alkyl;R4 is chosen from H, CN, COOR with R=Alkyl, Heteroalkyl, Aryl orHeteroaryl; substituted or unsubstituted Aryl, Heteroaryl, Alkyl mitC₁-C₂₀, Cycloalkyl mit C₃-C₂₀.

Examples of phenanthrolines to be used as n-doped ETM include:

Other ETM include metal complexes, such as metal chelates. A form of themetal chelates are metal quinolates and quinoxalines. Some materials arethose with the structure:

where M is chosen from Ti, Zr, Hf, Nb, Re, Sn and Ge,each R is independently chosen from hydrogen, C₁-C₂₀-Alkyl,C₁-C₂₀-Alkenyl, C₁-C₂₀-Alkinyl, Aryl, Heteroaryl, Oligoaryl,Oligoheteroaryl, Oligoarylheteroaryl, —OR_(x), —NR_(x)R_(y), —SR_(x),—NO₂, —CHO, —COOR_(x), —F, —Cl, —Br, —I, —CN, —NC, —SCN, —OCN, —SOR_(x),SO₂R_(x), where R_(x), and R_(y) are chosen from C₁-C₂₀-Alkyl,C₁-C₂₀-Alkenyl and C₁-C₂₀-Alkinyl.

Examples of quinoxalines include:

Other ETM include compounds according to the following formulae:

where R₁, R₂, R₃, and R₄ are in each occurrence independently selectedfrom H, halogen, CN, substituted or unsubstituted aryl, heteroaryl,alkyl, heteroalkyl, alkoxy and aryloxy.

Examples of such ETM are:

Hole transport materials (HTM) that are used as host for the p-dopantsinclude phenylamines, triphenyl-amines, fluorenes, benzidines.

Examples of such HTM include:4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA),4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2-TNATA),MeO-TPD (N,N,N′,N′-tetrakis(4-methoxy-phenyl)benzidine),(2,2′,7,7′-tetrakis-(N,N-diphenylamino)-9,9′-spirobifluoren (spiro-TTB),N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine,N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-9,9-spiro-bifluorene,9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorine,N,N′-bis(phenanthren-9-yl)-N,N′-bis(phenyl)-benzidine,2,2′-bis[N,N-bis(biphenyl-4-yl)amino]9,9-spiro-bifluorene,1,3,5-tris{4-[bis(9,9-dimethyl-fluorene-2-yl)amino]phenyl}benzene, andtri(terphenyl-4-yl)amine;N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPD).

The p-dopant can have a reduction potential in the range of 0V vs.Fc/Fc+ to 0.4V vs. Fc/Fc+. Fc/Fc+, as usual the Ferrocene/Ferroceniumredox couple. Reduction potentials can be considered as measures for theLUMO of a molecule.

Examples of p-dopants include:

Name Chemical name MW OA-1 2,2′-(perfluorocyclohexa-2,5-diene-1,4- 276(F4TCNQ) diylidene)dimalononitrile OA-2(perfluoronaphthalene-2,6-diylidene)dicyanamide 314 OA-3N,N′-bicyano-2,5-dichloro-1,4-chinodiimine(2,5-dichloro- 2613,6-difluorocyclohexa-2,5-diene-1,4-diylidene)dicyanamide OA-4N,N′-bicyano-2,5-dichloro-3,6-difluoro-1,4- 225chinodiimine(2,5-dichlorocyclohexa-2,5-diene-1,4- diylidene)dicyanamideOA-5 N-(2,3,5,6-tetrafluoro-4-iminocyclohexa-2,5- 203dienylidene)cyanamide OA-6 1,4,5,8-Tetrahydro-1,4,5,8-tetrathia-2,3,6,7-384 tetracyanoanthrachinone OA-7 1,3,4,5,7,8-Hexafluoronaphtho-2,6- 362chinontetracyanomethane OA-8 3,6-bis(cyano(4-cyano-2,3,5,6- 586tetrafluorophenyl)methylene)-2,5-difluorocyclohexa-1,4-diene-1,4-dicarbonitrile OA-92,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2- 651(perfluorophenyl)acetonitrile) OA-102,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2- 1095(perfluorobiphenyl-4-yl)acetonitrile); OA-112,2′,2″-(Cyclopropane-1,2,3-triylidene) tris (2-(2,6- 672dichloro-3,5-difluoro-4-(trifluoromethyl) phenyl)acetonitrile); OA-122,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2-(2,6- 902dichloro-3,5-difluoro-4-(trifluoromethyl)phenyl)-acetonitrile) Somedopants have MW > 300. Some compounds have a MW > than 500.

Asymmetric phenanthrolines can be used as an electron transport layer inthe devices of the described subject matter. Asymmetric phenanthrolinescan also be used when they are n-doped with dopants that are, or thatform, neutral radicals (or, e.g., diradicales, their dimers, oligomers).

The dopants that are, or that form, neutral radicals (or, e.g.,diradicales, their dimers, oligomers) can form stable layers when usedas dopants in a matrix having asymmetric phenanthrolines.

Metal quinoxalines can be used as electron transport materials dopedwith dopants that are, or that form, neutral radicals (or, e.g.,diradicales, their dimers, oligomers). Precursor dopants can form stablelayers when used as dopants in a matrix having metal quinoxalines.

Diazapyrenes can be used as electron transport materials doped withdopants that are, or that form, neutral radicals (or, e.g., diradicales,their dimers, oligomers). Precursor dopants can form stable layers whenused as dopants in a matrix having metal quinoxalines.

Stability, low voltage, and high efficiency can be achieved in deviceswhere organic mesomeric compounds are used as organic p-doping agentsfor the doping of an organic semiconductive hole transport matrixmaterial. The organic mesomeric compound can be a radialene compoundwith the following formula:

in which each X is

where each R₁ is independently selected from aryl and heteroaryl andaryl and heteroaryl are at least partially or completely substitutedwith electron acceptor groups.

Examples of emitter materials include Fluorescent emitters such as4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB); CBP, antracene, Metal chelates such as 3 quinoline Aluminum(Alq3); Phosphorescent emitters such as Ir-chelates; Ir(ppy)3 Fir-pic.

Emitter materials can be mixed with an emitter host. The host can alsocontribute to the emission. Examples of emitter hosts include:3,9-di(naphthalen-2-yl)perylene+3,10-di(naphthalen-yl)perylene mixture(DNP); and NPD.

FIGS. 1 a and 1 b are energy diagrams of a transparent OLED in oneembodiment of the described subject matter without doping. The positionof the energy levels are shown in the upper part (HOMO and LUMO) withoutexternal voltage and in the lower part with applied external voltage. Inthis embodiment, both electrodes have the same work function. Here, forthe sake of simplicity, the blocking layers 4 and 6 are also shown.

FIG. 2 b is an energy diagram of a transparent OLED with dopedcharge-carrier transport layers and matching blocking layers accordingto an embodiment of the described subject matter. Note the band bendingadjacent to the contact layers, here of ITO in both cases.

FIG. 3 shows the luminance vs. voltage curve of the embodiment presentedin example 1; the monitor luminance of 100 cd/m 2 is attained already at4 V. The efficiency is 2 cd/A. However, here, no transparent contact(e.g., ITO) is used as anode material. The transparent contact issimulated by a semitransparent (e.g., 50%) gold contact. Thus, this is asemitransparent OLED.

FIG. 4 shows the normalized luminance over time in an accelerated agingtest (current density=60 mA/cm²). The figure compares the bottomemitting device with the transparent device. It can be seen that thedevices have the same behavior; the measured points almost completelyoverlap each other. The extrapolated lifetime is in excess of 10,000 h.

FIG. 5 shows the comparison of the optical transmittance of an exemplarydevice of the described subject matter (102) compared with an existingdevice (103). The device 102 exhibits superior transmittance incomparison with the device 103. Note that the transmittance was measuredthrough the glass substrate and through the encapsulation substrate andis greater than 70% in the visible range and greater than 75% between460 nm and 800 nm. In contrast, the device 103 exhibits a transmittanceless than 62% and largely less than 50% of a larger range of the visiblespectrum. The transmission spectra of the device 103 is also morewavelength dependent (i.e., the spectra is less flat and has strongercolor). The transmittance of the glass substrate with the ITO (101) isalso shown for comparison purposes.

In the embodiment shown in FIGS. 1 a-b, no space charge zone occurs atthe contacts. This embodiment has a high energy barrier for thecharge-carrier injection. This high energy barrier, under certaincircumstances, cannot be overcome or overcome with difficulty when usingavailable materials. Hence, the injection of charge carriers from thecontacts is less effective. The OLED shows an increased operatingvoltage.

According to the described subject matter, increased performance isachieved, in some embodiments, by transparent OLEDs with doped injectionand transport layers, optionally in combination with blocking layers.FIG. 2 a shows one exemplary arrangement. In this embodiment, thecharge-carrier-injecting and conducting layers 3 and 7 are doped, sothat space charge zones are formed at the interfaces to contacts 2 and8. The doping is sufficient to allow for the space charge zones to beeasily tunneled through. Such doping has been shown to be possible forthe p-doping of the hole transport layer for non-transparentlight-emitting diodes (e.g., X. Q. Zhou et al., Appl. Phys. Lett. 78,410 (2001); J. Blochwitz et al., Organic Electronics 2, 97 (2001)).

The foregoing arrangements exhibit various characteristics: (1)increased injection of charge carriers from the electrodes into thedoped charge-carrier transport layers; (2) independence from thedetailed preparation of the charge-carrier-injecting materials 2 and 8(e.g., (I) injection layers may not be required if doping is used; (II)the layers which contact the electrodes may not need “special” treatmentto improve injection (such as annealing, surface modification of ITO,etc); (III) arrangements such as inverted structures with the ETL on thebottom electrode, i.e., cathode on the substrate or non-invertedstructures can be created without great constraints); (3) gives theoption of choosing, for the electrodes 2 and 8, materials havingcomparatively high barriers for the charge-carrier injection (e.g., thesame material in both cases such as ITO).

EXAMPLES Example 1

In the following example, the electron transport layer is not yetn-doped with stable large organic dopants. An embodiment with thenonstable n-doping of a known electron transport material(Bphen=bathophenanthroline) with Li demonstrates the effectiveness ofthe transparent OLED with doped organic transport layers (U.S. Pat. No.6,013,384 (J. Kido et al.), applied for on Jan. 22, 1998; J. Kido etal., Appl. Phys. Lett. 73, 2866 (1998)). This approximately 1:1 mixtureof Li and Bphen demonstrates the effectiveness of the doping. This layeris not stable thermally and operationally. It is assumed that themechanism of doping is different because of the high dopingconcentration. On doping with organic molecules and doping ratios ofbetween 1:10 and 1:10000, it can be assumed that the dopant does notsignificantly affect the structure of the charge-carrier transportlayer. Where the concentration is 1:1 of doping metals, e.g., Li, thesame cannot be assumed.

The OLED in Example 1 has the following layer structure (invertedstructure):

-   -   1 a Substrate, e.g., glass;    -   2 a Cathode: ITO as purchased, untreated;    -   3 a n-Doped electron-transporting layer: 20 nm Bphen:Li, 1:1        molecular mixing ratio;    -   4 a Electron-side blocking layer: 10 nm Bphen;    -   5 a Electroluminescent layer: 20 nm Alq 3, may be mixed with        emitter dopants in order to in-create the internal quantum yield        of the light production;    -   6 a Hole-side blocking layer: 5 nm triphenyldiamine (TPD);    -   7 a p-Doped hole-transporting layer: 100 nm Starburst m-MTDATA        50:1 doped with F 4-TCNQ dopant (thermally stable to about 80        degrees C.);    -   8 a Transparent electrode (anode) indium tin oxide (ITO).

The mixed layers 3 and 7 are prepared by a vapor deposition process invacuo by mixed evaporation. In principle, such layers can also beprepared by other processes as well, such as, e.g., vapor deposition ofthe substances one upon the other, followed by a possiblytemperature-controlled diffusion of the substances into one another; orby another type of deposition (e.g., spin-on deposition) of the alreadymixed substances in or outside of vacuum. The blocking layers 3 and 6are likewise vapor-deposited in vacuo, but can also be prepared byanother process, e.g., by spin-on deposition in or outside of vacuum.

FIG. 3 shows the luminance vs. voltage curve of a semitransparent OLED.For test purposes, a semitransparent gold contact (e.g., 50%transmission) was used. For a luminance of 100 cd/M 2 an operatingvoltage of 4 V is used. This value represents a low operating voltagefor transparent OLEDs, especially those with an inverted layerstructure. This OLED demonstrates the feasibility of the describedsubject matter. Because of the semitransparent cover electrode, theexternal current efficiency is limited to a value of about 2 cd/A, shortof 5 cd/A as expected for OLEDs with pure Alq3 as the emitter layer.

Devices of the described subject matter demonstrate increasedefficiency, lifetime, and transparency and decreased voltage.

The described devices can be fabricated more easily and reliably thanexisting OLEDS. The use of doping layers allows for directly depositingtransparent conductive oxides over a charge carrier transport layerwithout the necessity of buffer light absorbing buffer layers such asCuPc or metal layers. Also, a multi-step deposition procedure for theITO is not necessary.

The following examples demonstrate the features of the OLEDs of thedescribed subject matter. For reference, the OLED performance iscompared with bottom emitting OLEDs that are made in the same batch. Thebottom emitting OLEDs are produced in parallel with the transparentOLEDs. The difference is that, on the bottom emitting devices, Al isdeposited as a cathode instead of ITO.

Using a non-optimized structure, a variation of the ETM doped with ED-8demonstrates some favorable ETMs. The structure was made on Glass/ITOsubstrate with the following layer sequence: 50 nm of NPD p-doped withOA-11; 10 nm of NPD as EBL; emitter host doped with 0.5 wt % of rubrene;10 nm of ETM-6 as HBL; the ETM in the following table doped with ED-8followed by 100 nm of ITO.

Voltage increase at 10 mA/cm{circumflex over ( )}2, Doping compared tothe reference (e.g., ETM concentration the reference cathode is Ag) Alq310    2 V ETM-4 8 1.8 V ETM-6 8 0.8 V ETM-11 8 0.8 V ETM-6 10% + 1 nm0.4 V pure dopant ETM-6 10% + 1 nm 0.4 V pure dopant

The following layer sequence is a non-optimized OLED structure which wasused for the experiments (the thickness is given in parenthesis):

ITO (90 nm)

NPD (50 nm) doped with 3 wt % of OA-11

NPD (10 nm)

DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

ETM-6 (10 nm)

ETL=n-doped ETM (70 nm)

top ITO (100 nm)

The comparative bottom emitting devices have a 100 nm Aluminum layer inplace of the top ITO layer.

A variation of the n-doping concentration of the ETL demonstrates afavorable doping concentration. The structure was made on Glass/ITOsubstrate with the following layer sequence: 50 nm of NPD p-doped withOA-11; 10 nm of NPD as EBL; emitter host doped with 0.5 wt % of rubrene;10 nm of ETM-6 as HBL; the ETM: dopant system in the following table;followed by 100 nm of ITO.

Voltage increase at 10 mA/cm{circumflex over ( )}2, ETM: dopant Dopingcompared to the reference (e.g., system concentration the referencecathode is Ag) ETM-4: ED-8 8 1.53 V  ETM-6: ED-8 2 2.7 V ETM-6: ED-8 41.5 V ETM-6: ED-8 8 0.9 V ETM-11: ED-14 8 0.8 V ETM-11: ED-8 8 1.1 VETM-11: ED-14 10 0.76 V 

It can be seen in the table above that the optimum doping concentrationto achieve a low voltage, with a comparative voltage increase of lessthan 1 V, compared to the bottom emitting device, is higher than orequal to 8%. A dopant concentration greater than 25% is less desirablein the ETL. However, a highly doped buffer can be used in addition tothe doped ETL.

Another embodiment includes a thin (e.g., 1 to 15 nm thick) highly dopedcharge transport layer at the interface of the top electrode (this layerbeing localized between the light-emitting region and the electrode).Another embodiment includes a thin (e.g., 0.5 nm to 3 nm) pure dopantlayer as a buffer layer at the interface of the top electrode (betweenthe charge transport layer and the top electrode).

ED-8:doped ETM with 1 nm ED-8 interlayer

Voltage increase at Voltage increase at 10 mA/cm², compared to the 100cd/m², compared to the reference (e.g., the reference reference (e.g.,the reference ETM cathode is Al) cathode is Al) ETM-4 1 0.75 ETM-6 0.430.30 ETM-9 0.39 0.31 ETM-11 0.19 0.26 ETM-4 0.84 0.59 ETM-6 0.47 0.27ETM-9 0.67 0.44 Voltage at 10 Voltage at 10 Voltage at 10 mA/cm² forED-8 mA/cm² for ED-14 mA/cm² for ED-8 ETM (100 cd/m²) (100 cd/m²) (100cd/m²) ETM-4 (2.72) ETM-6 2.77 (2.4) 2.58 (2.27) ETM-9  3.09 (2.63) 2.76(2.42) 2.6 (2.28) ETM-11 3.9 (3.5) 2.55 (2.79)  ETM-40 2.9 (2.69)

Similar results as those with ED-14 were obtained with ED-3 and ED-4.

The devices of the described subject matter exhibit a good life-timebehavior. The time before the device exhibits half of the initialbrightness can be more than 10,000 h, under accelerated aging (See FIG.4).

Comparative Examples

Comparative devices were constructed according to known techniques,without using doped layers. The anode (e.g, ITO) was treated with oxygenplasma before the deposition of the organic layers, to enhance the holeinjection. A thin layer of Mg:Ag with an atomic ratio of 40:1 wasdeposited, as part of the cathode (e.g., electron injection layer), ontop of the organic layers. A sputtered ITO layer followed the thin metallayer.

The performance of the comparative device is poor, even if the sameorganic stack is used. The comparative devices exhibit a voltage (at acurrent density of 10 mA/cm2) more than 1 V higher. The (cd/A)efficiency is reduced due to the additional absorption of the thin metallayer. The overall power efficiency is further reduced because of theadditional effects of the absorption of the metal layer and theincreased operating voltage.

The samples with doped layers exhibit a higher yield, especially thesamples using the described diazapyrenes, asymmetrical phenanthrolines,and metal quinoxalines as doped ETM.

The comparative devices exhibited a low yield. Many included shortcircuits immediately after being produced. The cause is believed to bedue to metal diffusion and sputter damage during metal and ITOdeposition. the doped layers can improve the robustness of the device,not only against the sputtering process. By using doped layers (withorganic doping), the yield and device efficiency can be higher, e.g.,because these layers offer protection against sputtering. The dopingeffect can be stable and strong such that even after sputtering, thedevice performs well.

Example Embodiment

An example OLED has the following layer structure (non-invertedstructure):

Substrate, glass

Anode, ITO (90 nm)

doped hole transport layer, NPD (50 nm) doped with 3 wt % of OA-11

non doped interlayer NPD (10 nm) (optionally an electron blocking layer)

Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

non doped interlayer E™-6 (10 nm) (optionally a hole blocking layer)

electron transport layer, ETM-6 n-doped with 10 wt % ED-14 (70 nm)

Cathode, ITO (100 nm)

Another example OLED with an inverted structure has:

Substrate, glass

Cathode, ITO (e.g., 100 nm)

electron transport layer, ETM-6 n-doped with 4 wt % ED-14 (40 nm)

non doped interlayer E™-6 (10 nm) (optionally a hole blocking layer)

Emitter layer DNP:Alq3:DCJTB in the ratio 70:29:1 (20 nm)

non doped interlayer NPD (10 nm) (optionally an electron blocking layer)

doped hole transport layer, NPD (80 nm) doped with 8 wt % of OA-11

Anode, ITO (90 nm)

FIG. 6 shows the luminance vs. voltage curve of a transparent OLEDaccording to an embodiment with a non-inverted structure. For aluminance of 100 cd/m² an operating voltage of 2.14 V is used. Thisoperating voltage is one of the lowest voltages for transparent OLEDs.

The high transparency and the flatness of the optical transmittance ofthe inventive OLEDs are especially useful for white OLEDs. White OLEDswere constructed by different methods, such as mixing multiple emittersin one light-emitting region, or stacking OLEDs through so-calledconnecting units.

The use of doped layers according to the described subject matter makesit possible to attain nearly the same low operating voltages and highefficiencies in a transparent structure as occur in a traditionalstructure with one-sided emission through the substrate. This is due, asdescribed, to the efficient charge-carrier injection, which, thanks tothe doping, is relatively independent of the exact work function of thetransparent contact materials. In this way the same electrode materials(or, e.g., transparent electrode materials of only slightly differentwork functions) can be used as electron-injecting contacts andhole-injecting contacts.

From the examples and the knowledge of one ordinarily skilled in theart, it is obvious to a person skilled in the art that manymodifications and variations of the described subject matter arepossible which fall within the scope of the described subject matter.For example, transparent contacts other than ITO can be used as anodematerials (e.g., as in H. Kim et al., Appl. Phys. Lett. 76, 259 (2000);H. Kim et al., Appl. Phys. Lett. 78, 1050 (2001)). Furthermore, someembodiments include transparent electrodes made by combining asufficiently thin intermediate layer of a nontransparent metal (e.g.,silver or gold) and a thick layer of the transparent conductivematerial. In that case, the thickness of the intermediate layer is thinenough so that the device is still transparent (e.g., 75% transparent inthe entire visible spectral region). Because of the thick dopedcharge-carrier transport layers, no damage to the light-emitting layersis to be expected during sputter. A further embodiment uses, for thedoped electron transport layer, a material whose LUMO level is too deep(in the sense of FIGS. 1 a-b and 2 a-b layers 7 or 3 a) to be able toefficiently inject electrons into the blocking layer and light-emittinglayer (6 or 4 a, and 5 or 5 a, respectively) (thus, greater barriersthan those shown in FIG. 2 a). In that case, it is possible to usebetween the n-doped electron transport layer (7 or 3 a) and blockinglayer (6 or 4 a) or the light-emitting layer (5 or 5 a) a thin (2.5 nm)layer of a metal having a lower work function than the LUMO level of thedoped transport layer. The metal layer is thin enough so that theoverall transparency of the component is mostly maintained (see L. S.Hung, M. G. Mason, Appl. Phys. Lett. 78, 3732 (2001)).

1. A transparent, thermally stable light-emitting component havingorganic layers, comprising: a transparent substrate; a transparentanode; a hole transport layer adjacent to the anode; at least onelight-emitting layer; a charge-carrier transport layer for electrons;and a transparent cathode, wherein the transparency in the visiblespectral region is at least 75%, wherein the hole transport layer isp-doped with an acceptor organic material and the electron transportlayer is n-doped with a donor organic material, and the molecular massesof the dopants are each greater than 200 g/mole, and wherein thetransparent, thermally stable light-emitting component having organiclayers is an organic light-emitting diode.
 2. A light-emitting componentaccording to claim 1, further comprising: at least one of a hole-sideblocking layer located between the doped hole transport layer and thelight-emitting layer or an electron-side blocking layer located betweenthe doped electron transport layer and the light-emitting layer.
 3. Alight-emitting component according to claim 1, further comprising: aelectrode layer located between the anode and the hole transport layerand a electrode layer located between the charge-carrier transport layerand the cathode.
 4. A light-emitting component according to claim 1,wherein the doping concentration of the organic dopants is such that anohmic injection takes place from the anode into the charge-carriertransport layer or from the cathode into the hole transport layer.
 5. Alight-emitting component according to claim 3, wherein the electrodelayers comprise indium tin oxide (ITO) or a degenerate oxide other thanITO.
 6. A light-emitting component according to claim 1, wherein thecathode includes a metallic intermediate layer adjacent to the subjacentdoped, charge-carrier transport layer when the cathode is located on topor the anode includes a metallic intermediate layer adjacent to thesubjacent doped, hole transport layer when the anode is located on topand wherein the metallic layer has a nominal thickness between 0.1 nmand 3 nm.
 7. A light-emitting component according to claim 1, wherein nometal layer is located between the doped hole transport layer and theanode when the anode is on top or between the doped electron transportlayer and the cathode when the cathode is on top.
 8. A light-emittingcomponent according to claim 1, where the anode and cathode are locatedbetween the substrate and encapsulation cover and the transparency is atleast 70% for each wavelength between at least 400 nm and 800 nm.
 9. Alight-emitting component according to claim 1, wherein the molarconcentration of admixture in the hole transport layer or in theelectron transport layer or in both the hole transport layer and theelectron transport layer is in the range of 1:100,000 to 1:10,calculated on the ratio of doping molecules to main-substance molecules.10. A light-emitting component according to claim 1, wherein the molarconcentration of admixture in the hole transport layer or in theelectron transport layer, or in both the hole transport layer and theelectron transport layer, is at least 1 wt %, calculated on the ratio ofdoping molecules to main-substance molecules.
 11. A light-emittingcomponent according to claim 2, wherein the thickness of each of thehole transport layer or the electron transport layer, of thelight-emitting layer and of the at least one of a hole-side blockinglayer or an electron-side blocking layer lies in the range of 0.1 nm to50 μm.
 12. A light-emitting component according to claim 1, wherein thecathode is in direct contact with a doped transport layer and is facingaway from the substrate when the cathode is on top or the anode is indirect contact with a doped transport layer and is facing away from thesubstrate when the anode is on top and wherein the doped transport layeris a hole transport layer or an electron transport layer.
 13. Alight-emitting component according to claim 1, wherein the organicn-dopant material is selected from the group consisting of heterocyclicradicals, diradicals, dimers, an oligomer, a polymer, a dispirocompound, and a polycycle thereof, having the structure according to oneof the following formulae:

wherein structures 3 and 4 have one or more cyclic linkages A and/or A1and/or A2, wherein A, A1 and A2 are selected from the group consistingof carbocyclic, heterocyclic, polycyclic ring systems, and anycombination thereof, which may be substituted or unsubstituted,

wherein A1 and A2 are present individually or together and A1 and A2 areselected as in structures 3 and 4 and T=CR22, CR22R23, N, NR21, O or S,and

wherein structure 7 has one or more bridge bonds Z and Z1, Z or Z1, Z1and Z2, or Z1 or Z2, and Z, Z1 and Z2 are independently selected fromthe group consisting of alkyl, alkenyl, alkynyl, cycloalkyl, sililyl,alkylsililyl, diazo, disulphide, heterocycloalkyl, heterocyclyl,piperazinyl, dialkyl ether, polyether, primary alkylamine, arylamine,polyamine, aryl, and heteroaryl.
 14. A light-emitting componentaccording to claim 1, wherein the organic acceptor organic material is aquiniode derivative or a triylidene derivative, with a reductionpotential in the range of 0V vs. Fc/Fc+ to 0.4V vs. Fc/Fc⁺.
 15. Alight-emitting component according to claim 12 or 13, wherein then-doped, donor organic material is an asymmetrically substitutedphenanthroline with the following structure

wherein: R1 and R2 are selected from the group consisting of substitutedor unsubstituted Aryl, Heteroaryl, and Alkyl; and R3 is selected fromthe group consisting of H, CN, substituted or unsubstituted Aryl,Heteroaryl, and Alkyl; R4 is selected from the group consisting of H,CN, COOR with R=Alkyl, Heteroalkyl, Aryl or Heteroaryl, substituted orunsubstituted Aryl, Heteroaryl, Alkyl mit C1-C20, and Cycloalkyl mitC3-C20.
 16. A light-emitting component according to claim 12 or 13,wherein the n-doped, donor organic material has the structure:

wherein M is selected from the group consisting of Ti, Zr, Hf. Nb, Re,Sn and Ge, each R is independently selected from the group consisting ofhydrogen, C₁-C₂₀-Alkyl, C₁-C₂₀-Alkenyl, C₁-C₂₀-Alkinyl, Aryl,Heteroaryl, Oligoaryl, Oligoheteroaryl, Oligoarylheteroaryl, —OR_(x),—NR_(x)R_(y), —SR_(x), —NO₂, —CHO, —COOR_(x), —F, —Cl, —Br, —I, —CN,—NC, —SCN, —OCN, —SOR_(x), SO₂R_(x), and where R_(x) and R_(y) areselected from the group consisting of C₁-C₂₀-Alkyl, C₁-C₂₀-Alkenyl, andC₁-C₂₀-Alkinyl.
 17. A light-emitting component according to claim 12 or13, wherein the n-doped, donor organic material has the structure:

wherein R₁, R₂, R₃, and R₄ are independently selected from the groupconsisting of H, halogen, CN, substituted or unsubstituted aryl,heteroaryl, alkyl, heteroalkyl, alkoxy, and aryloxy.
 18. Alight-emitting component according to claim 1, wherein the anode isbetween the substrate and the at least one light-emitting layer.
 19. Alight-emitting component according to claim 1, wherein the cathode isbetween the substrate and the at least one light-emitting layer.
 20. Alight-emitting component according to claim 3, wherein the electrodelayers include different transparent contact materials.
 21. Alight-emitting component according to claim 1, further comprising acontact-improving layer located between the electron transport layer andcathode and a contact-improving layer located between the anode and thehole transport layer, wherein the contact-improving layers areconfigured not to prevent charge from passing through.
 22. Alight-emitting component according to claim 1, further comprising acontact-improving layer located between the electron transport layer andcathode or a contact-improving layer located between the anode and thehole transport layer, wherein the contact-improving layers areconfigured not to prevent charge from passing through.
 23. Alight-emitting component according to claim 1, wherein thelight-emitting layer includes a mixed layer of several materials.
 24. Alight-emitting component according to claim 1, wherein the p-doped holetransport layer includes a mixture of an organic main substance and anacceptor doping substance and an acceptor doping substance and themolecular mass of the dopants is greater than 200 g/mole.
 25. Alight-emitting component according to claim 1, wherein the electrontransport layer includes a mixture of an organic main substance and adonor doping substance and an acceptor doping substance and themolecular mass of the dopants is greater than 200 g/mole.
 26. Alight-emitting component according to claim 1, wherein when thetransparent cathode is on top, the transparent cathode includes atransparent protective layer or when the transparent anode is on top,the transparent anode includes a transparent protective layer.
 27. Alight-emitting component according to claim 1, wherein when thetransparent cathode is on top, the transparent cathode includes ametallic intermediate layer adjacent to the subjacent dopedcharge-carrier transport layer or when the transparent anode is on top,the transparent anode includes a metallic intermediate layer adjacent tothe subjacent doped hole transport layer, wherein the transparency ofthe metal intermediate layer in the visible spectral region is at least75% and the thickness of the metal intermediate layer is between 0.3 nmand 3 nm.
 28. A light-emitting component according to claim 1, whereinthe sequence of p-doped hole transport layer and transparent anode isrepeated.
 29. A light-emitting component according to claim 1, whereinthe sequence of n-doped electron transport layer and transparent cathodeis repeated.
 30. A light-emitting component according to claim 2,further comprising a metallic electron-injection-promoting layer locatedbetween the doped electron transport layer and either the electron-sideblocking layer or the light-emitting layer, wherein the transparency ofthe metallic electron-injection-promoting layer in the visible spectralregion is at least 75%.